1. Field of the Invention
The present invention relates to an apparatus for controlling an array antenna and a method therefor, and in particularly, to an apparatus for controlling an array antenna comprising a plurality of antenna elements arranged in a predetermined arrangement configuration and a method therefor.
2. Description of the Related Art
FIG. 6 shows a conventional phased array radar apparatus disclosed in Japanese Patent Laid-Open Publication No. 63-167287.
Referring to FIG. 6, an array antenna 1 comprises a plurality of natural number M of antenna elements 100-1 to 100-M, which are, for example, aligned, wherein each of transmission and reception modules RM-1 to RM-M respectively connected to the antenna elements 100-1 to 100-M comprises a circulator 2 used as an antenna combiner for commonly using one antenna element for reception and transmission, a receiver 3 having a frequency converter and a demodulator, an analog-to-digital converter (hereinafter, referred to as an A/D converter) 4, a phase shifter 5 for shifting a phase of a transmitting signal by a set amount of phase shift, and a high-frequency high output transmitting power amplifier (hereinafter, referred to as a high output power amplifier) 6 for amplifying and transmitting a high-frequency transmission signal.
A transmitting pulse divider and distributor circuit 101 divides a transmitting pulse, which is sent from an oscillator circuit (not shown) in a form modulated using a predetermined pulse modulation method, into a plurality of M subpulses, and then outputs the plurality of M subpulses to respective phase shifters 5 of the transmission and reception modules RM-1 to RM-M, respectively. On the other hand, information of target azimuth and distance is inputted to a transmitting beam control circuit 102. The control circuit 102, based on the inputted information, calculates respective amounts of phase shift for respective phase shifters 5 of the transmission and reception modules RM-1 to RM-M, and then outputs the same to respective phase shifters 5 of the transmission and reception modules RM-1 to RM-M, respectively. In this state, if a transmitting pulse is radiated toward a target object, the radiated transmitting pulse impinges on the target object and then is thereby reflected. After the resulting reflected signal is received by the array antenna 1, the reflected receiving signals received by the antenna elements 100-m are respectively inputted into the receivers 3 through the circulators 2, are respectively demodulated so as to obtain intermediate frequency signals by the receivers 3, and further the demodulated signals are respectively converted into a receiving digital signals R1 to RM by the A/D converters 4.
A distributor circuit 400 divides and distributes the receiving digital signals R1 to RM respectively outputted from respective transmission and reception modules RM-1 to RM-M into a plurality of N sets of digital signals, each set of digital signals including a plurality of N digital signals, and then outputs respective distributed N sets of digital signals to first to N-th beam forming circuits 500-1 to 500-N, respectively. Each of these beam forming circuits 500-1 to 500-N, using the receiving digital signals R.sub.1 to R.sub.M, controls their amplitude and phase with a predetermined manner, thereby forming beams of receiving signals in their respective desired directions and then outputting the same as a plurality of N beams of receiving signals B.sub.1 to B.sub.N. In this case, the beam forming circuits 500-1 to 500-N perform a process for eliminating effects of unnecessary radio waves which come up in directions other than the direction of the target object, and then extracts only reflected radio waves sent from the target object, further detects the direction, the distance, and the like of the target object.
In a method for eliminating unnecessary radio waves used in the above-mentioned conventional apparatus, as shown in FIG. 7, an auxiliary beam of radio signal formed by a pair of antenna elements is superimposed on a main beam of radio signal formed by all the antenna elements so that the phase of the auxiliary beam of radio signal is reverse to the main beam of radio signals, whereby the main beam of radio signal is directed toward the incoming direction of the desired radio wave and also the zero point of the radiation pattern is formed in an incoming direction of an unnecessary radio wave.
The phases of the transmitting signals are controlled by the phase shifters 5, while the receiving signals are subjected to beam formation by converting the analog signals received by respective antenna elements 100-m into the digital signals. This process is performed because of the following reasons. That is, since the transmitting radio signals must be radiated to a distant target object, it is necessary to amplify the transmitting signals with the high output power amplifier 6.
FIG. 8 shows input and output characteristics of the conventional high output power amplifier 6. As is apparent from FIG. 8, to make more efficient use of the high output power amplifier 6, the amplifier's saturation region in which its amplification factor becomes constant should be used. In other words, since the amplification factor of the high output power amplifier 6 is used at a constant value, it becomes possible to control only the phase. Accordingly, upon the transmission, it is not necessary to convert the analog transmitting signals into any digital signals, however, the phase of the transmitting radio signals are controlled by the phase shifters 5.
The control apparatus for the above-mentioned conventional phased array radar apparatus is principally purposed for application to radars, and therefore, the difference between the frequencies of the receiving and transmitting radio signals has not been taken into his consideration. However, in satellite communications or the like, generally speaking, the frequency of the receiving frequency is different from that of the transmitting frequency by about 10% thereof. If the above-mentioned conventional method is applied to this case as it is, the phase of the transmitting radio signal can not be adaptive controlled based on the receiving radio signal. This leads to the following disadvantageous problems: for example,
(a) the main beam of radio signal can not be directed toward the desired direction; and
(b) large effects of unnecessary radio waves such as interference radio waves leads to misdirection in the control apparatus.
Further, as shown in the conventional apparatus, elimination of unnecessary radio waves has been implemented only to the receiving signals. In the above-mentioned conventional radar apparatus or the like, it is necessary only to radiate a strong radio wave to the target object, namely, it is necessary only to radiate the transmitting radio signals only in the predetermined directions. However, in the satellite communications, it is necessary to receive the transmitted radio signals without any distortion, and therefore it is necessary to provide a communication line having a better signal to noise power ratio. If, upon the reception, the zero point of the radiation pattern is formed in the incoming directions of the unnecessary radio waves, it is necessary to radiate the transmitting radio signals in the same radiation pattern as that of the reception.